Hydrostatic power steering system

ABSTRACT

A hydrostatic power steering system including an electric motor is provided for a automotive vehicle. The hydrostatic power steering system utilizes a power steering piston responsive to movement of the rack element of the rack and pinion steering connection. The power steering piston is provided in a piston chamber divided by a valve land into first and second portions, each of which are connected to first and second hydraulic lines. A torque sensor is connected to the steering shaft for providing an output torque signal to a power steering controller. The electric motor of the system is responsive to a command signal generated by the power steering controller. An electrically operated valve is connected to the hydraulic lines and is adapted to receive a signal from the power steering controller for controlling flow of hydraulic fluid to the power steering chamber. The electrically operated valve arrangement is rendered inoperative within a predetermined zone defined by the command signal and the output torque signal.

BACKGROUND OF THE INVENTION

The present invention relates to a hydraulic and electric power steeringcontrol system for a motor vehicle and more particularly, forcontrolling the steering angle of the wheels to be steered of thevehicles in accordance with the state of movement of the motor vehicleto which the system is applied.

Various types of steering apparatuses for motor vehicles have beendeveloped. Such examples include electric motor-driven pump-type powersteering systems in which a conventional power steering system uses anelectric motor as a power source to drive an oil pump so as to providefor hydrodynamic power steering. The flow rate of the oil delivered fromthe oil pump is controlled to allow a driver to operate a steering wheelwith optimum steering force.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a new andimproved steering apparatus which is capable of performing vehiclesteering with high response and control accuracy while at the same timeensuring reliability of operation and precise control of the vehicle towhich the power steering system is provided.

In accordance with the present invention, there is provided ahydrostatic power steering system including an electric motor andcomprising a pinion fixedly connected to a steering shaft and meshinglyconnected with a rack of the steering mechanism. A hydraulic pump isprovided and a power steering piston, responsive to movement of therack, is positioned in a power steering chamber. The chamber is dividedby a valve land into first and second portions, each of which areconnected to first and second hydraulic lines. At least a pair ofvehicle wheels are connected to the rack and piston by a linkagemechanism. A torque sensor arrangement is connected to the steeringshaft for providing an output torque signal to a power steeringcontroller and the electric motor is responsive to a command signalgenerated by the power steering controller. A fluid reservoir isprovided for containing hydraulic fluid utilized in the hydraulicportion of the steering system. Electrically operated valves areprovided in the steering system and connected to each of the first andsecond hydraulic lines and adapted to receive a signal from the powersteering controller for controlling flow of hydraulic fluid to the powersteering chamber. The electrically operated valves are inoperativewithin a predetermined zone defined by the command signal and the outputtorque signal.

A further object of the present invention is to provide the powersteering controller with a command computing circuit and a motor controlcircuit. The output torque signal and a vehicle speed signal aretransmitted to the command computing circuit and the motor controlcircuit outputs the command signal to control the electric motor.

Another object of the present invention is to provide a hydrostaticpower steering system in which the predetermined zone of inoperationincreases at increased vehicle speed when a ratio of the command signalcurrent is divided by the output torque signal so as to define a line ofminimum slope. The predetermined zone of inoperativeness will decreaseat decreased vehicle speed when the ratio of the command signal currentdivided by the output torque signal defines a line having a maximumslope within the predetermined zone.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and features of the present invention will become morereadily apparent from the following detailed description of thepreferred embodiments taken in conjunction with the accompanyingdrawings in which:

FIG. 1 is the first embodiment of the hydrostatic electric powersteering system;

FIG. 1(A) is a second embodiment of the hydrostatic electric powersteering system;

FIG. 2 is a block diagram of the first embodiment of the hydrostaticelectric power steering system;

FIG. 3 is a power assist gain schedule of the first embodiment of thehydrostatic electric power steering;

FIG. 4 is a third embodiment of the hydrostatic electric power steeringsystem;

FIG. 5 is a fourth embodiment of the hydrostatic electric power steeringsystem;

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a configuration of the hydrostatic electric power steeringsystem. Therein, a steering wheel 5 is connected via steering shaft 33to a pinion gear 8 fixedly attached to the steering shaft. The piniongear 8 is in meshing engagement with the rack 7 of the known type ofrack and pinion steering. A hydraulic pump is indicated at referencenumeral 2 and is of the bidirectional type so as to provide hydraulicfluid, under pressure, to through pump output conduits 2a. An electricmotor 1 operates the pump 2. A reservoir 13 is provided for supplyingthe hydraulic fluid to the pump 2 and for receiving the workinghydraulic fluid back into the reservoir from a drain system to bediscussed later. The hydraulic fluid is pumped, under the action of thedriving electric motor 1 and bidirectional oil pump 2 from the reservoirthrough an oil filter 12. Hydraulic conduit 9f provides the fluid to apair of hydraulic lines 9c and 9d. In normal operation, the hydraulicfluid is pumped from one chamber to another. Fluid is supplied from thereservoir when the fluid level is low due to leakage or other flowproblems. A check valve 10 is located in hydraulic line 9d as is a checkvalve 11 in hydraulic line 9c. The check valves 10 and 11 may be of anyconventional structure which would permit flow in a first direction andblock flow in the opposite direction. The hydraulic fluid in lines 9cand 9d is provided to first and second power steering chambers 9a and9b, respectively. A valve land 9e separates the two chambers from oneanother. The hydraulic lines 9d and 9c are connected to the chambers 9aand 9b, respectively. The power steering system 9 is integral with therack 7 and is connected, at either end, through a steering linkage 7b tothe wheels 7a, to be steered. A pair of hydraulic return lines 13a and13b are interconnected with hydraulic lines 9d and 9c. The hydraulicreturn lines are normally closed by pressure relief valves 3 and 4,respectively. The pressure relief valves may be of any known type whichwould open under conditions of overpressure in lines 9c and 9d so as topermit the return of the hydraulic fluid from hydraulic lines 9c and 9dthrough the return lines 13a and 13b to the reservoir 13. Of course, thepressure relief valves would again shut-off flow of the hydraulic fluidthrough the hydraulic return lines upon the pressure in the systemreaching equilibrium.

With reference to FIG. 1(A), the solenoid valve 29 of FIG. 1 has beenomitted. The solenoid valve 29 functions to return the wheels of thevehicle to the straight ahead position when the driver has removed hishands from the steering wheel after a turning operation. However, valve29 is only needed when the reduction ratio is very large or the oil pumpdoes not allow reversible action. In the first case, the reaction forcefrom the tires cannot move the rack since internal friction of theelectric motor and pump are increased by the high reduction ratio. Inthe second case, the rack cannot move as the oil circuit is lockedbecause the oil pump is not rotated by oil flow.

With reference to FIG. 2, a power steering controller 14 is provided forcontrolling the electric motor 1 which drives the hydraulic pump 2. Thepower steering controller comprises a command computing circuit 14a anda motor control circuit 14b. In order to provide control variables tothe power steering controller 14, a vehicle speed sensor 32 is providedwhich transmits a signal indicative of the vehicle speed to the commandcomputing circuit 14a. A torque sensor 6 connected with the steeringshaft 33 also provides a signal which is indicative of the torqueapplied to the steering shaft to the command computing circuit 14a. Thevalues received by the command computing circuit 14a are transmitted tothe motor control circuit 14b. The motor control circuit is connected tothe electric motor 1 so as to control operation thereof and, thereby,control operation of the bidirectional oil pump 2.

With reference to FIG. 3, a power assist gain schedule of the firstembodiment is discussed. Therein, it can be seen that at a relativelyhigh speed of the vehicle, the torque sensor output is greater than atlow vehicle speed and the current command from the command computingcircuit is greater at lower torque sensor output. Accordingly, atrelatively high vehicle speed, a predetermined zone is provided in whicha solenoid valve 29 (to be discussed later) is held in an open conditionand the electric drive motor 1 is not actuated. The zone ofinoperativeness of the solenoid valve 29 is relatively large and theslope of a line therethrough is relatively small. The slope iscalculated by dividing the torque sensor output into the currentcommand. Conversely, at relatively low vehicle speeds, the predeterminedis small and the slope of a line, calculated in the foregoing manner, islarger.

With respect to the operation of the embodiment of FIG. 1, when thevehicle is moving in a straight line, power assist of the steering isunnecessary. In this condition, the solenoid valve 29 is open and theright chamber and left chamber 9b and 9a, respectively, are connectedthrough the solenoid valve 29. The power steering controller provides aninput signal to the solenoid valve 29 to control the opening or closingof the solenoid valve. When a driver of the vehicle turns the steeringwheel 5 during movement of the vehicle and the torque sensor output iswithin the predetermined zone, the solenoid valve 29 remains open andthe electric motor 1 is not driven. Once the torque sensor outputexceeds the threshold of the predetermined zone, the solenoid valve 29closes and the electric motor 1 is actuated. As discussed, the operationof the solenoid valve is dependent upon the output from the controller14 based upon the torque sensor and vehicle speed sensor inputs thereto.

During normal power steering operation, when the driver of the vehicleturns the steering wheel 5, the torque applied to the steering shaft 33builds up. The torque is detected by the torque sensor 6 and the torquesignal is provided to the power steering controller 14. The commandcomputing circuit 14a in the power steering controller 14 calculates thecurrent command signal to the motor control circuit and transmits asignal to the solenoid valve 29 to open. The motor control circuit 14bprovides the control signal to the electric motor. The oil pump 2,driven by the electric motor 1, will pump oil into one chamber 9a or 9bfrom the other chamber of the hydraulic cylinder. The oil flow will pushagainst the power steering piston 9 and function as a hydraulic powerassist in the steering of the vehicle. The control signal to theelectric motor is based on a torque feedback control type in view of thetorque sensor 6.

The oil pump 2 functions as a reduction gear on the hydrostatic electricpower steering system. On the conventional electric power steering, asdiscussed above, the reduction ratio is usually around 10. Thehydrostatic electric power steering allows a much greater reductionratio using a large power steering piston diameter and a high speedsmall pump. In the conventional electric power steering system, themotor is operated at around 1000 rpm at maximum power so as to obtainthe reduction ratio of 10. In contrast, the hydrostatic electric powersteering motor of the present invention can be operated at about 3000rpm or more at maximum power. This permits the hydrostatic electricpower steering motor of the present invention be much smaller than themotors in the conventional power steering systems.

It is desirable for the steering wheel and steering system to return tothe straight position when the driver removes his or her hands from thesteering wheel or grips the steering wheel with a light relaxed grip.This phenomena is caused by the force applied to the rack which isgenerated by suspension geometry. In these situations, the torqueapplied to the steering shaft is within the predetermined zone and thesolenoid valve is open. Thus, the force applied to the rack returns thesteering to the straight-ahead position.

Under certain conditions, an electric power system can fail. Suchconditions would be a lack of power for the electric motor, motorlock-up or pump lock-up. If such a failure of the electric power systemof the present invention occurs, the driver of the vehicle can stillsafely operate the vehicle. If, for example, the driver wanted to steerto the right, oil would flow from the right to the left chamber, i.e.,from chamber 9b to 9a, through the solenoid valve 29. Accordingly, thedriver can steer the same as regular steering without power assist.Additionally, when the system experiences a pressure overload aspreviously discussed, the spring loaded pressure release valves open,and the steering system again functions in a non-power assist mode ofoperation.

In the second embodiment of the invention and in all subsequentlydiscussed embodiments of the invention, like reference numerals will beutilized to identify like elements previously discussed.

The second embodiment of the invention is substantially similar to thatdiscussed with regard to the preceding embodiment. A difference existsin the provision of a pair of normally open solenoid valves 29 and 30which are arranged in the hydraulic circuit as follows. The pair ofsolenoid valves 29 and 30 replace the solenoid valve 29 of the previousembodiment. A difference in operation of the two embodiments is in theprovision of the two solenoid valves which permit the hydraulic fluid toreturn to the reservoir 13 instead of to the other chamber of thecylinder in which the power steering piston 9 is located. Each of thesolenoid valves 29 and 30 receive signals from the power steeringcontroller 14 in the manner discussed with respect to the embodiment ofFIG. 1. However, each of the solenoid valves 29 and 30 are locatedbetween the hydraulic lines leading to the chamber of the power assiststeering arrangement and the return line to the reservoir. Accordingly,when the driver would want to steer to the left, the hydraulic fluidwould flow from the right side of the chamber, i.e., side 9b through thehydraulic line 9c, the solenoid valve 30 and the return line 13b so asto drain back to the reservoir 13.

In the embodiment of FIG. 5, the bidirectional oil pump 2 of the firstembodiments has been replaced by a unidirectional oil pump 36. In orderto provide for the flow of hydraulic fluid to each side of the powersteering chamber in which the power steering piston 9 is located, athree position solenoid valve 35 is provided. The solenoid valvereceives its operational control from the power steering controller 14in the manner of the previously discussed embodiments. The solenoidvalve 29, for controlling the flow of the hydraulic fluid from one sideof the chamber to the opposite side of the chamber remains the same asin the first embodiment and is likewise controlled by the power steeringcontroller 14 in the same manner as in the first embodiment.

In the embodiment of FIG. 5, a pump outlet conduit 36a is provided whichextends from the outlet of the pump to the three position solenoid valve35. The three position solenoid valve 35 is designed so as to providefluid flow to one side of the chamber 9a or 9b dependent upon steeringconditions. As shown in the drawing, hydraulic fluid is discharged fromthe pump 36 through the outlet conduit 36a through the three positionsolenoid valve 35 and into the right hand side 9b of the power steeringcylinder. The solenoid valve 29 would again function in the same manneras previously discussed so as to permit outflow of the fluid through thereturn line 36b leading to the reservoir 13 When moved to its lowermostposition, the three position solenoid valve 35 provided fluid to theleft side of the power steering piston 9 in the chamber section 9a anddrains fluid from the right side 9b of the chamber back to the reservoir13. In all other respects, operation of the embodiment of FIG. 5 is thesame as the foregoing discussions with regard to FIGS. 1 and 4.

While the presently preferred embodiments of the present invention havebeen shown and described, it is to be understood that this disclosure isfor the purpose of illustration and that various changes andmodifications may be made without departing from the scope of theinvention as set forth in the appended claims.

What is claimed is:
 1. A hydrostatic power steering system including anelectric motor comprising:a pinion fixedly connected to a steering shaftand meshingly connected with a rack; a bidirectional hydraulic pumpdriven by said electric motor; a power steering piston responsive tomovement of said rack positioned in a power steering chamber, saidchamber being divided by a land into first and second portions, saidfirst chamber portion being connected to a first hydraulic line and saidsecond chamber portion being connected to a second hydraulic line, saidfirst and second hydraulic lines being connected to the bidirectionalhydraulic pump so that hydraulic fluid is alternatively pumped to saidfirst hydraulic line or said second hydraulic line, at least a pair ofwheels connected to said rack and piston by a linkage mechanism; torquesensor means connected to said steering shaft for providing an outputtorque signal to a power steering controller, said electric motor beingresponsive to a command signal generated by said power steeringcontroller; a reservoir for providing hydraulic fluid to said first andsecond hydraulic lines.
 2. The hydrostatic power steering system ofclaim 1, further comprising:a hydraulic feed line for supplying saidhydraulic fluid to said first and second hydraulic lines.
 3. Thehydrostatic power steering system of claim 2, wherein said hydraulicpump is an oil pump, and including:a first check valve positioned insaid first hydraulic line; a second check valve positioned in saidsecond hydraulic line; and oil pump outlet conduit connected from saidoil pump to said first and second hydraulic lines at a locationdownstream of said first and second check valves.
 4. The hydrostaticpower steering system of claim 1, further comprising:a first hydraulicreturn line connected to said first hydraulic line and said reservoir; asecond hydraulic return line connected to said second hydraulic line andsaid reservoir; first and second pressure relief valves positioned insaid first and second hydraulic return lines respectively, said pressurerelief valves being responsive to overpressure in said first and secondhydraulic lines to reduce said overpressure by opening and permittingsaid hydraulic fluid to return to said reservoir.
 5. The hydrostaticpower steering system of claim 1, wherein said power steering controllercomprises a command computing circuit and a motor control circuit. 6.The hydrostatic power steering system of claim 5, wherein said outputtorque signal and a vehicle speed signal are transmitted to said commandcomputing circuit, and said motor control circuit outputs said commandsignal to said electric motor.
 7. The hydrostatic power steering systemof claim 1, including a first outlet conduit having one end connected tosaid hydraulic pump and an opposite end connected to said firsthydraulic line, said first outlet conduit being continually incommunication with said first chamber portion.
 8. The hydrostatic powersteering system of claim 7, including a second outlet conduit having oneend connected to said hydraulic pump and an opposite end connected tosaid second hydraulic line, said second outlet conduit being continuallyin communication with said second chamber portion.
 9. The hydrostaticpower steering system of claim 1, further comprising:electricallyoperated valve means connected to each of said first and secondhydraulic lines and adapted to receive a signal from said power steeringcontroller, said electrically operated valve means being inoperativewithin a predetermined zone defined by said command signal and saidoutput torque signal.
 10. The hydrostatic power steering system of claim9, wherein said electrically operated valve means is a normally opensolenoid valve.
 11. The hydrostatic power steering system of claim 9,wherein said predetermined zone increases at increased vehicle speedwhen a ratio of the command signal divided by said output torque signaldefines a line of minimum slope, and said predetermined zone decreasesat decreased vehicle speed when the ratio of said command signal dividedby said output torque signal defines a line of maximum slope.
 12. Thehydrostatic power steering system of claim 9, wherein said electricallyoperated valve means is a pair of normally open solenoid valves.
 13. Thehydrostatic power steering system of claim 12, wherein a first one ofsaid pair of normally open solenoid valves is located in a firsthydraulic return line and a second one of said pair of normally opensolenoid valves is located in a second hydraulic return line.
 14. Thehydrostatic power steering system of claim 9, wherein said hydraulicpump is an oil pump, and including:an outlet conduit having a first endconnected to said oil pump and a second end connected to a threeposition solenoid valve, said first and second hydraulic lines beingconnected to said three position solenoid valve; and a return conduitconnected to said three position solenoid valve and said reservoir.